In recent years, as a result of the trend towards decreases in the weight of automobiles, efforts have been proceeding at obtaining high strength steel members, such as to achieve a tensile strength of 590 MPa, in order to reduce the weight thereof. Due to this trend, with thin steel sheets which are widely used in automobiles, as the strength of steel sheets has increased, press formability has decreased, and it is becoming difficult to manufacture complicated shapes. Specifically, as the strength of a steel sheet increases, its ductility decreases, and problems occur such as breakage of the sheet in locations undergoing a high degree of working, an increase in springback and wall warp, and a worsening of dimensional accuracy.
Accordingly, with high strength steel sheets and particularly those having a tensile strength of 780 MPa or higher, it is not easy to manufacture parts by means of press forming as a method of cold working. If roll forming is used instead of press forming, it may be possible to perform working on the above-described high strength steel sheets, but roll forming can only be applied to a part having a uniform cross section in the longitudinal direction, so the shapes of parts to which roll forming can be applied are limited.
As disclosed in UK Patent No. 1,490,535, in a hot press forming method in which a heated steel sheet is press formed, since the steel sheet is heated to a high temperature so that the material of the steel sheet softens and becomes highly ductile before forming, the steel sheet in a heated state can be formed into a complicated shape with good dimensional accuracy. In addition, by use of the die cooling technique in which a steel sheet is heated to a temperature in the austenite region before press forming in dies and then rapidly cooling in the dies, it is possible to simultaneously achieve an increase in strength of the steel sheet by martensitic transformation, i.e., by quenching.
However, die cooling in a hot press forming method has the problems that the quenching hardness of the resulting formed member is not inadequate and remains at the level of Hv 400-490 in the case of a 0.2% C steel material, and that the hardness of the formed member extremely varies locally.
Advanced Materials and Processes, vol. 146, No. 6, 12/94, page 16 discloses hot press forming technology developed by Plunger, a Swedish company. Die quenching (rapid cooling in dies) from 980° C. is described therein. It is assumed that the die temperature is from room temperature to several tens of degrees C. since there is no description of heating for the dies.
JP 08-269615 A1 discloses a hot rolled steel sheet for rapid quenching which comprises C: 0.18-0.30%, Si: 0.01-1.0%, Mn: 0.2-1.5%, P: at most 0.03%, S: at most 0.02%, sol. Al: at most 0.08%, Cr: 0.1-0.5%, B: 0.0006-0.0040%, N: at most 0.01%, optionally at least one of Cu: at most 0.5%, Ni: at most 0.3%, and Ti: 0.01-0.05%, and a remainder of iron. This steel sheet is given a high strength by high frequency hardening after it is cold worked.